qa: test_sync_io: zero buffers prior to read
qa: test_sync_io: zero buffers prior to read

Be extra paranoid to ensure we read back the correct data and didn't
just find it in undefined memory.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph